The <<pot life>> of a thermosetting composition is in most cases accompanied by the onset of an unwanted polymerization reaction of said composition. This polymerization reaction onset is often disregarded in relation to the polymerization reaction occurring during the final curing stage, for example for a well tubing or a pipe. Now, this thermosetting composition curing stage is conditioned by the time left before gelation of said composition (this time is referred to in the description hereafter as <<residual gel time>> under the temperature conditions of said implementation. In fact, the gelation of a thermosetting composition, i.e. the change from the viscous liquid state to the viscoelastic solid state, marks an irreversible change in its rheological behaviour, its viscosity becoming infinite then. Gelation can also be defined by the following change: before gelation, the composition is soluble in a solvent consisting of the initial monomers, and after gelation the composition is no longer soluble in this solvent. Since the viscosity rise is relatively sudden at the approach of the gel point, the specialist's knowledge of the residual gel time during the pot life is critical as regards the applications considered. For example, in the case of resins intended for impregnation of fiber mats for composite parts, the residual gel time of the thermosetting composition is often defined for controlled temperature conditions. If any deviation from these conditions occurs, the thermal history of the drums can no longer be controlled: the residual gel time of the thermosetting composition is no longer accessible and the viscosity properties for impregnation are no longer controlled, which is harmful to the impregnation process. In an extreme case, the residual gel time may be too short for the application considered. Thus, in the case of flexible preforms pre-impregnated with polymerizable resin intended for the inner lining of wells, as described in patent application WO-98/59,151, setting of the lining on the well walls by radial deformation towards the outside is conditioned by the viscosity of the polymerizable resin, which depends on the progress stage of the polymerization reaction. Above the gel point, the preform loses its flexibility and cannot be applied by radial deformation on the well walls. It is thus impossible to use the thermosetting resin, conditioned by the progress of the polymerization, above the gel point.
Control of the use of thermosetting compositions therefore requires consideration of the thermal history preceding this use.
The conventional gel time measuring methods are based on rheological measurements which follow the evolution with time of the dynamic shear moduli G′ and G″ (Winter H. H., Polymer Engineering and Science, 1987, Vol.27, 1698-1702). Certain rudimentary devices detecting an infinite viscosity increase by means of a mobile body moved in the thermosetting composition give satisfactory measurements of the residual gel time, but all these equipments involve sampling. Now modern industrial practices very often use non-destructive material evaluation techniques which ideally allow to monitor and to control certain properties in-situ, i.e. while the materials are used.
Dielectrometry has thus emerged as a prime non-destructive technique allowing real-time monitoring of the evolution of the dielectric properties of thermosetting compositions during polymerization, in particular during curing stages and use of pure or composite resins, on the laboratory scale as well as on the industrial scale (Stéphan F., Boiteux G., Seytre G., Ulanski J., J. Non Crystalline Solids, 1994, Vol.172-174, pp.1001-1011). It is well-known that a network develops progressively through linear growth, branching and crosslinking leading to the formation of molecular species of ascending size. As regards the dielectric response of the material, polymerization manifests itself for example in a decrease in the conductivity measured in the medium. Correlating a conductivity calculated from the loss factor with the degree of polymerization of the material has also been proposed (Kranbuehl D. E., Processing of Composites, pp.137-157, Hanser Ed., 2000), as well as using the real and imaginary parts of the complex impedance of epoxy-amine resins cured at high temperatures to calculate the resistivity of the resin and to correlate it with the progress of the reaction, i.e. the crosslinking degree of the network (Mijovic J., Yee C. F. W., Macromolecules, 1994, Vol.27, pp.7287-7293). It is furthermore well-known that dielectric measurements can also be correlated with certain physical properties of the resin such as its viscosity before the gel point.
It is however admitted that dielectric measurements (assembled under the generic term dielectrometry) cannot provide a direct measurement of the residual gel time of a resin under given temperature conditions, since the gelation of a thermosetting composition induces no particular signature on said dielectric measurements (Eloundou J. P., Gérard J. F., Pascault J. P., Boiteux G., Seytre G., Die Angewandte Makromolekulare Chemie, 1998, Vol.263, pp.57-70). In fact, the gelation of a thermosetting composition is defined by a chain growth degree such that a macromolecule of infinite size has formed. At gel point, the formation of a non densely crosslinked 3D network structure is therefore not a sensitive disturbance on the dipoles and charge carriers scale.
Surprisingly enough, the applicant has however found that it is possible to correlate by dielectrometry, for a given temperature, the residual gel time of a thermosetting composition at any time whatever the thermal history of said composition, and more particularly during or after its pot life, or during its use.